This invention relates to airborne antennas and, more particularly, to such antennas providing multiple beam excitation usable with anti-jam adaptive processing to suppress jamming and interference.
A variety of antennas have been made available for reception of Global Positioning System (GPS) signals for navigational and other purposes. A more critical objective than the mere capability to receive such signals, is the objective of enabling reception in the presence of interference or jamming signals. Interference may be the unintended result of reception of signals radiated for some purpose unrelated to GPS operations. Jamming, on the other hand, may involve signals intentionally transmitted for the purpose of obstructing reception of GPS signals. In airborne operations which are dependent upon use of GPS signals, deleterious effects of interference or jamming may be particularly disruptive.
For reception via a fixed-position antenna in the presence of interference signals incident from a fixed azimuth, for example, a reduced-gain antenna pattern notch aligned to suppress reception at the appropriate azimuth may be employed as an effective solution. However, for airborne operations a more complex solution is required. With an aircraft and its antenna operable in a variety of geographical locations and conditions, with constantly changing azimuth orientation during flight, interference or jamming signals may be incident from any azimuth and with constantly changing azimuth. At the same time, maneuvers such as banked turns of an aircraft, for example, tilt the aircraft and its antenna so that the interference or jamming signals may be incident from different and changing elevation angles.
A variety of adaptive processing techniques have previously been described. Such techniques typically provide an anti-jam capability based on provision of reduced-gain antenna pattern notches and alignment of such notches at the incident azimuth of undesired incoming signals. However, to enable practical employment of such techniques for reception of GPS signals under critical airborne operations, reliable, low-profile antennas providing a multi-beam capability suitable for anti-jam application are required.
Size constraints regarding aircraft antennas may limit the implementation of anti-jam techniques in the context of aircraft-mounted GPS antennas. For example, at present many military aircraft are equipped with a fixed radiation pattern antenna (“FRPA”) for GPS operation. This small size antenna (i.e., active antenna volume in inches of 3.73×3.73×0.86 height, within a radome) provides no multi-pattern capability to support adaptive processing for anti-jam operation. Available antennas generally do not enable such adaptive processing capabilities to be implemented in an antenna of that size.
Examples of prior antennas providing anti-jam capabilities in the context of airborne GPS antennas are provided in U.S. Pat. Nos. 6,618,016 and 6,819,291, having a common assignee with the present application. The former patent describes, in particular, GPS antennas including four bent monopoles in combination with four slot elements to provide primary and auxiliary antenna patterns usable for aircraft anti-jam applications. The latter patent describes, in particular, GPS antennas including a circular array of eight monopole elements arranged to provide anti-jam capabilities.
Airborne applications may include large aircraft, smaller fighter and drone aircraft where small antenna size is important, and smaller objects such as missiles, guided bombs and other projectiles. In the latter categories of applications size, weight, cost and complexity become increasingly important, along with antenna anti-jam operational capabilities. For such applications, it is desirable to provide smaller antennas able to meet overall objectives of small size and low weight, cost and complexity, with concurrent high performance and the capability of providing multiple auxiliary antenna patterns usable for anti-jam adaptive processing for such applications.
Accordingly, objects of the present invention are to provide new and improved GPS antennas, and antennas which may have one or more of the following characteristics and capabilities:                effective basic GPS reception;        anti-jam capabilities;        GPS plus anti-jam in small configuration;        omnidirectional hemispherical primary coverage;        up to seven selectively usable auxiliary radiation patterns;        multiple pattern excitation suitable for adaptive processing anti-jam operation;        tuned elements capable of very small size implementation;        low cost, high reliability implementation; and        
small size, low profile configuration suitable for replacement of GPS aircraft antennas lacking anti-jam features.